There are numerous methods for manufacturing gear wheels. In chip-producing soft pre-machining, one distinguishes hobbing, gear shaping, generating planing and power skiving. Hobbing and power skiving are so-called “continuous” methods, as shall be explained further below.
In the chip-producing manufacturing of gear wheels, one distinguishes between the intermitted indexing process or single indexing process and the continuous method, which may also be called a continuous indexing process or face-hobbing.
In the continuous methods, for example, a tool comprising corresponding cutters is used to cut the flanks of a work piece. The work piece is cut as required in one continuous clamp, i.e., in an uninterrupted process. The continuous method is based on complex coupled movement sequences, in which the tool and the work piece to be machined perform a continuous indexing movement relative to each other. The indexing movement results from the driving in coordination with coupled driving of plural axle drives of a corresponding machine.
In the single indexing process, one tooth gap is machined, then, for example, a relative movement of the tool occurs, a so-called “indexing” movement (indexing rotation), in which the work piece rotates relative to the tool, and then the next tooth gap is machined. In this process, a gear wheel is manufactured step by step.
The initially mentioned gear shaping method may be described or represented by a cylinder gear transmission, because the intersection angle between the rotation axis R1 of the shaping tool 1 and the rotation axis R2 of the work piece 2 amounts to zero degree, as represented schematically in FIG. 1. The two rotation axes R1 and R2 run parallel when the intersection angle amounts to zero degree. The work piece 2 and the shaping tool 1 rotate continuously about their rotation axes R2 and R1, respectively. In addition to the rotational movement, the shaping tool 1 carries out a stroke movement, which is referenced in FIG. 1 by the double arrow shx, and removes chips from the work piece 2 during this stroke movement.
A skiving process was developed many years ago, the basics of which are approximately 100 years old. An early patent application on this subject, DE 243514, dates back to 1912. After the initial considerations and investigations, skiving was no longer seriously pursued further. Previously, complex processes, which were partly empirical, were necessary in order to find a suitable tool geometry for the skiving process.
In the 1980s, power skiving was developed. However, it was not until present-day simulation methods and modern machine CNC-controls, that the principle of power skiving could be implemented as a productive, reproducible and robust method. The high durability of present-day tool materials, the enormous high static and dynamical rigidity, and the high performance of the synchronous running of the modern machines now also aid the process.
In the skiving process, as shown in FIG. 2, there is an intersection angle (also called intersection angle of axes) Σ between the rotation axis R1 of the skiving tool 10 (also called a skiving wheel) and the rotation axis R2 of the work piece 20, which is different from zero. The resulting relative movement between the skiving tool 10 and the work piece 20 is a helical movement, which can be decomposed into a rotational portion (rotatory portion) and an advance portion (translational portion). A generation helical type gear transmission can be considered as a drive technology-specific analogon, wherein the rotational portion corresponds to the rolling and the advance portion corresponds to the gliding of the flanks. The greater the absolute value of the intersection angle Σ, the more the translational movement portion required for the machining of the work piece 20 increases. It causes namely a movement component of the cutting edges of the skiving tool 10 in the direction of the tooth flanks of the work piece 20. Thus, during power skiving, the gliding portion of the combing relative movement of the mutually engaging gear wheels of the equivalent helical gear is utilized to carry out the cutting movement. In power skiving, only a slow axial feed (also called axial feed) is required and the so-called shaping, or pushing, movement, which is typical for the gear shaping, is dispensed with. Thus, a return stroke movement also does not occur in power skiving.
The cutting speed during skiving is influenced directly by the rotational speed of the skiving tool 10 with respect to the work piece 20 and the utilized intersection angle Σ between the rotation axes R1 and R2. The intersection angle Σ and thus the gliding portion should be selected such that for a given rotational speed an optimum cutting speed is achieved for the machining of the material.
The skiving process is not utilized solely for the machining of outer toothings, as shown, e.g., in FIG. 2. When manufacturing inner toothings, skiving is significantly more productive than gear shaping or broaching, which have been used thus far.
Skiving can be utilized both in pre-toothing prior to heat treatment of the work piece 20 and in finishing-toothing after heat treatment. That is, skiving is suitable both for soft machining and for hard (fine) machining.
The movement sequences and further details of an established skiving process can be taken from the schematic representation in FIG. 2 that has already been mentioned. FIG. 2 shows skiving of an outer toothing on a cylindrical work piece 20, where the work piece 20 and the tool 10 (here a cylindrical skiving tool 10) rotate in opposite directions.
Additional relative movements also occur. An axial feed sax is required in order to machine the entire toothing width of the work piece 20 with the tool 10. If helical toothing is desired on the work piece 20 (i.e. β2≠0), a differential feed sD is superimposed onto the axial feed sax. A radial feed sD may be carried out as a lining movement. The radial feed srad may also be employed in order to influence the convexity of the toothing of the work piece 20.
In skiving, the vector of the cutting speed vc results substantially as the difference of the two velocity vectors vo and v2 of the rotation axes R1, R2 of the tool 10 and the work piece 20. The velocity vectors are tilted with respect to each other by the intersection angle Σ. The symbol vo is the velocity vector at the periphery of the tool 10 and v2 is the velocity vector at the periphery of the work piece 20. The cutting speed vc of the skiving process may thus be changed by the intersection angle Σ and/or the rotation speed in the equivalent helical gear. The axial feed sax has only a small influence on the cutting speed vc, which can be neglected and is thus not shown in the vector diagram comprising the vectors vo, v2 and vc in FIG. 2.
The skiving of an outer toothing of a work piece 20 using a conical skiving tool 10 is shown in FIG. 3. In FIG. 3 again, the intersection angle Σ, the cutting speed vc, the velocity vectors vo at the periphery of the tool 10 and v2 at the periphery of the work piece 20 as well as the inclination angle β0 of the tool 10 and the inclination angle β2 of the work piece 20 is shown. Here, in contrast to FIG. 2, the inclination angle β2 is different from zero. The two rotation axes R1 and R2 do not intersect, but are arranged skewed with respect to each other. For a conical skiving tool 10, the calculation point AP is usually chosen on the joint plumb of the two rotation axes R1 and R2, because tilting of the skiving tool 10 to provide end relief angles is not necessary. The calculation point AP coincides with the so-called “contact” point. The rolling circles of the equivalent helical generation gear contact each other at the calculation point AP.
In skiving, a tool 10 comes to application, which comprises at least one geometrically determined cutting edge. The cutting edge/cutting edges are not shown in FIG. 2 and FIG. 3.
The tool is of great importance in skiving. In the example shown in FIG. 2, the skiving tool 10 has the shape of a spur-toothed spur wheel. The outer contour of the base body in FIG. 2 is cylindrical. However, it can also be tapered (also called conical), as shown in FIG. 3. Because the tooth, or teeth, of the skiving tool 10 engages along the entire length of the cutting edge, each tooth of the tool 10 requires a sufficient end relief angle at the cutting edge.
An example of a single cutting tooth 3 of a straight-toothed conical skiving tool 10 is shown in FIG. 4A. The following statements also hold for helically toothed conical skiving tools 10. When starting from a conical skiving tool 10, it is obvious that the relief angle at the tooth head 4 (called the “head relief angle”) and at the tooth flank (called the “flank relief angle”) directly results from the shape of the cutting tooth 3. When considering a displacement from the tooth breast 5 in an axial direction, i.e., in the direction of R2, then the profile height steadily falls off. That is, the cutting tooth 3 gets progressively smaller along the axial direction. In FIG. 4A, the tooth breast 5 is in the lowest horizontal plane of the cutting tooth 3 and is therefore not visible. FIG. 4B shows a section B-B of the cutting tooth 3 where the tooth breast 5 is visible.
In the frontal regrinding of a conical skiving tool 10, the diameter of the head circle gets smaller. FIG. 4C shows a state after the regrinding in a schematic representation. The original shape of the cutting tooth 3 is characterized by the tooth breast 5 and the tooth head 4. The shape of the cutting tooth 3 after regrinding is characterized by the tooth breast 5′ and the tooth head 4′. The machine settings must be adapted after regrinding due to the resulting lowering of the diameter to the head circle.
When starting from a straight-toothed or a helically toothed cylindrical skiving tool 10, it is recognized that such a skiving tool 10 does not have so-called “constructional” relief angles, neither at the head nor at the flanks. If such a cylindrical skiving tool 10 is clamped in the conventional manner, there is no relief angle. Kinematic relief angles can be generated by tilting the skiving tool 10. In practice, tilting of the skiving tool 10 is achieved by an eccentric clamping of the skiving tool 10 in the machine, in order to offset the cutting face from the intersection point of axes (called cutting face offset). The contact point of the rolling circles of the skiving tool 10 and the work piece 20 no longer lies on the joint plumb of the rotation axes R1 and R2. The further the skiving tool 10 is tilted, the larger the effective relief angle becomes.
One problem that arises is that the life of known skiving tools 10 is partly unsatisfactory. If one of the cutting teeth 3 is excessively worn out or even damaged by an improper relative movement of the skiving tool 10 with respect to the work piece 20, then the manufacturing process must be interrupted and the skiving tool 10 exchanged. Such interruptions have a negative impact on productivity. In addition, tool cost increases when the skiving tool 10 must be reground or even exchanged.
It is a concern, then, to keep the cost of tools as low as possible by improving the lifetime of the tools.